The first three-fourths of "Thirst" is a snore, with little investigating and tons of commenting on Ducky's love life. We get to know Mary and she seems to be a wonderful person until the writers spring the trap. She's a psycho! That explains why the rest of the episode was so boring and uneventful.
I would probably like it more if the twist were any good, but it's awful. For the hundredth time, the love interest of a character turns out to be a bad guy. That Mary is completely crazy makes it worse. I know it's the ninth season and the writers may be getting short on ideas, but some creativity would be nice. How about happy relationships once in a while instead of the constant stream of dysfunctional ones?